A fuel injection system has been put into practical use, according to which a split fuel injection is carried out, namely fuel injection is carried out in an intake stroke and a compression stroke of a direct-injection engine. Various kinds of technologies have been proposed in order to increase controllability of the fuel injection in the compression stroke of the split fuel injection.
For example, according to a prior art of U.S. Pat. No. 7,069,138 (a direct-injection engine), a minimum injection time for a fuel injection valve is fixed depending on engine operation condition, such as fuel pressure, engine rotational speed and so on, and fuel injection is carried out in a compression stroke of the engine based on such minimum injection time.
According to another prior art, for example, as disclosed in International patent publication No. 2003-527527, a dynamic error of flow rate as well as a static error of the flow rate for a fuel injection valve is calculated based on an error of torque between respective cylinders as well as information of multiple operating points, so that a fuel injection amount is corrected based on such error of the flow rate in order to uniformize the torque among the cylinders. According to such prior art, it is proposed that the fuel injection amount is corrected by taking into consideration not only the static error of the flow rate in a condition of a full valve open of the fuel injection valve but also the dynamic error of the flow rate in a valve opening or in a valve closing operation of the fuel injection valve, so that a stable and homogeneous operation of the engine is obtained either in a stratified combustion mode or in a homogeneous combustion mode.
Fuel injection characteristic for a compression-stroke injection is shown in FIG. 12. An area, in which a fuel injection time is longer than a time “Tx” (which corresponds to a boundary point), is a linear area in which fuel injection amount is increased in proportion to an increase of the fuel injection time. An area, in which the fuel injection time is shorter than the time “Tx”, is a non-linear area in which the fuel injection amount varies with respect to the fuel injection time due to various factors, such as, individual differences and secular changes of the fuel injection valve and so on in FIG. 12, characteristics indicated by one-dot-chain lines show a range of variation of the fuel injection characteristic.
When compared the linear area with the non-linear area, the error of the variation for the fuel injection characteristic in the linear area is “E1”, while the error of the variation for the fuel injection characteristic in the non-linear area is “E2”. And the error “E1” is smaller than the error “E2” (E1<E2). This means that the variation of the fuel injection amount may become larger when the compression-stroke injection (the fuel injection of small amount of fuel) is carried out in the non-linear area. And thereby, the output of the engine as well as emission of exhaust gas may be adversely affected.
According to the above prior art (U.S. Pat. No. 7,069,138), when the compression-stroke injection is carried out in the non-linear area, the combustion stability may be decreased due to the variation of the fuel injection characteristic in the non-linear area, and thereby the emission of the exhaust gas may be adversely affected. In other words, according to this prior art, it may not be possible to absorb the variation of the fuel injection characteristic in the non-linear area, and unstable combustion may not be avoided.
According to the above other prior art (International patent publication No. 2003-527527), although it is possible to absorb the error of the fuel injection characteristic in the linear area (a normally operating area), it may not be possible to absorb the error of the fuel injection characteristic in the non-linear area. In other words, there is no guarantee that the error of the flow rate is treated as such error, which would be changed in a primary relationship with any one of engine operating parameters. As a result, there is a problem that the combustion stability may be decreased, the emission of the exhaust gas may be deteriorated and so on.